Ascorbic acid: vitamin C

ascorbic acid is comprised a a 5 member ring, including an ether. each Carbon atom of the ring has one functional group: an ester, 2 Hydroxyl groups, and an ethyl group, which itself contains 2 hydroxyl groups. Also, there is a double bond between the 2 hydroxyl Carbons of the ring. The molecule has 2 chiral centers occurring at adjacent carbons. Therefore, there are 2 sets of enantiomers, 1 of which ( R,R and S,S) are known as erythorbic acid, or isoascorbic acid (http://www.inchem.org/documents/jecfa/jecmono/v28je03.htm).

The other set of enatiomers, (S,R and R,S) are the stereoisomers more commonly referred to as ascorbic acid. L-ascorbic acid, or vitamin C, is naturally occurring and has great biological importance. It is an antioxidant, meaning it prevents the oxidation of other important molecules by being oxidized themselves. Interestingly, the entiomer of vitamin C (D-ascorbic acid), which is not found in nature, has the same antioxidant properties, but much less vitamin activity(http://ipac.kacst.edu.sa/eDoc/eBook/3761.pdf). This suggests that vitamin C serves biological functions which are stereospecific, which it does: Vitamin C serves as an enzyme cofactor. Among other functions, L-ascorbic acid is vital to the production of collagen, carnitine, and some neurotransmitters(Gropper SS, Smith JL, Grodd JL (2005). Advanced nutrition and human metabolism. Belmont, CA: Thomson Wadsworth. pp. 260–275).  
Enzymatic activity is often stereospecific because an enzymes active site must bind specifically to its substrate. Because of this specificity, a substrates enantiomer is often incapable of reacting with its enzymes, as shown by L and D ascorbic acid.